First: the literature is unequivocal about the effects of chronic sleep deprivation. This cellular aging article fits within what is known already. Sleep deprived humans are dumber, more stressed, and now age faster that non-sleep deprived humans.
The tacit assumption for work-hour restrictions is that sleep deprived physicians result in inferior medical care for patients. It turns out that the literature does not support the tacit assumption. When researchers looked at complications/mortality pre- and post- work-hour restriction, there was no difference. This result is somewhat counter-intuitive, and has been reproduced in various contexts. The one mortality benefit that has been for physicians themselves: less physicians get into car accidents after their call shifts. It's worth noting that there is literature to suggest that less ICU errors are made by sleepy residents, however, this also didn't translate into mortality differences.
So what's happening? There are a few interpretations of these results. The first is that resident physicians really aren't important for patient care, and that the majority of care provided comes from attending physicians. For anyone who has been admitted to an academic institution, this interpretation is silly and is unsatisfying.
The second is that there are sufficient checks and balances. When a sleepy physician makes a silly mistake, nurses can catch errors "this patient has an infection; are you sure you don't want to start antibiotics?" and pharmacists can catch errors "You ordered a hundred times the lethal dose. I'll correct that for you." When the day-team comes by and hears about the patient, they can quickly fix the errors made. Having made some silly errors myself, this certainly plays at least part of what happens.
The third is that most of medicine is rote. Frankly, after a few years of doing the same thing over and over again, you don't really need that much brain power to do a lot of medicine. I have admitted hundreds of people with brain tumors over the years, and the immediate workup and management for the majority for patients is identical. I can do this in a sleep-deprived state. I am not saying that some cases are more complicated than others. As a HN analogy: consider how much sleep you need to print "Hello World!" in your favorite language.
The fourth is that the ability to capture complications / mortality is increasing at the same rate as residency work hour restrictions are resulting in better patient care. In other words, we're searching and finding problems we wouldn't have caught otherwise. It's hard to counter this argument, but I don't know of any data for or against this position.
There are certainly more reasons, but I think the point is made. It's not obvious, it's multifactorial, but it's definitely robust.
OK, so the obvious question is this: Why do resident physicians need to work so hard?
First, let's look at the statistics. Suppose you're providing neurosurgical care for a catchment area of a million people. Brain tumors, as a category, affect 1:10,000 people. This means you'll be admitting 100 new brain tumors a year. However, not all brain tumors are the same (in fact, depending on how you want to slice the pie and what you consider a tumor, there are on the order of 200 different tumor diagnoses). Some tumors are common (e.g. metastatic lung tumors) and some are literally a one in a million diagnosis. In order to see the gamut of tumors in your training, you simply need the time in hospital. In my estimation, brain tumor surgery has a mean of 3.5 hours, with 95% of the cases being between 90 minutes and 24 hours. This means that the average academic center is doing at most two tumors per day. The argument here is that you need the hours to see the cases. There's no doubt that sleepier doctors learn worse than non-sleepy doctors. But I can tell you that I have participated in very rare operations in sleep deprived states, and I remember the approaches much better than common operations I've seen in non-sleep deprived states.
Suppose we are interested in maximizing physician well-being, regardless of the literature supporting patient outcomes. The simple solution is to simply restrict work hours. So, who runs the hospitals? One approach is to hire more residents. Well, residents eventually need jobs. Suffice it to say, swamping the medical profession with more doctors who have less training is not an intuitively satisfying solution to the problem. Moreover, more doctors means more handovers. Increasing handovers has been shown to result in order more unnecessary tests for patients, and to result in inferior patient care.
Restricting work hours means that surgeons may have to train longer. Well, at minimum in the US, becoming a neurosurgeon is 4 + 4 + 7 years of post-secondary education (undergraduate, medical school, residency -- residency used to be 6 years). Many residents choose to become subspecialists, adding another year or two (or possibly 3 or 4). Further work hour restrictions could turn the 15-year training process into a 16 or 17 year process. Fair enough, this could solve the problem. But really? From a purely economic perspective, this is silly. Think of the opportunity cost of educating yourself for 20 years (i.e. 20 years of debt) to work for 25 more before being asked to retire.
Suffice it to say, it's a tough problem. In recognition of this last point, many specialties are moving towards competency based training. That is, graduating surgeons when they meet a competency rather than having spent sufficient time in hospital. This is a no brainer in other fields (e.g. flying planes, building bridges). While this is in fact reflecting the times (greater emphasis on patient safety than in the past), there's no doubt in my mind that this is being implemented because those in power feel that they are graduating surgeons less competent than the generation before them.
This last point, is in my opinion, the important question to ask. If anyone knows of any literature about this last point, I'd be grateful.